Prior methods of replacing windows and other fenestration units in an existing structure, especially stucco covered structures, have included utilizing a flush flange window design, also described as a pocket replacement, where the frame of the pre-existing window is left in place while the sashes are removed from the pre-existing window assembly. The new window is then inserted from the exterior of the building and centered in the prepared opening, including the pre-existing window frame. The window flange is secured tight against the window frame, an exterior seal is created by sealing the flange to the stucco with sealant placed on the flanges of the new window, and an interior seal is created by sealing the window frame to the existing window assembly. A second, more involved methodology is also used where the exterior stucco is chipped away to reveal the pre-existing window frame and nailing fin and the entire pre-existing window assembly is removed prior installation of the new window assembly. Similarly methodology is employed with other types of fenestration units, such as pre-hung door assemblies. Improvements remain to be made over either method, where leaving the pre-existing frame in the rough opening can be problematic from aesthetic and weatherability standpoints and removal and replacement of the stucco surface is labor intensive.